The invention relates generally to color processing and, more particularly, to techniques for color characterization and transformation.
Since the introduction of the CIE (Commission International de l'Eclairage) color measurement system in the early 1930's, many different color spaces have been proposed for different applications. A color space, also referred to as a color “metric,” is essentially a coordinate system by which a color can be quantified.
A color space can be used to characterize the color output of a color imaging system relative to other color imaging systems. By characterizing multiple color imaging systems, the color space facilitates using different imaging systems to produce matching colors. An “ideal” color space would calculate a color mapping between different color imaging systems that achieves an acceptable color match between the systems without subjective or empirical adjustment.
Color spaces differ in the parameters expressed on their coordinate axes and the manner in which the parameters are calculated. CIE color spaces use CIE Standard Observer functions that are based on color matching functions and result in a unique set of tristimulus values XYZ for any color measured under specified conditions. The tristimulus values XYZ are calculated from the spectral output of either an additive or subtractive color system convoluted with the response function of either a 2 degree or 10 degree Standard Observer. In the case of reflective hard copy, the spectral reflectance curve is typically convoluted with a standard illuminant to estimate the expected spectral output of the reflective color.
One CIE color space is the CIELAB color space. In this color space, L* represents lightness, a* represents redness-greenness, and b* represents yellowness-blueness. The CIELAB color space employs a modified von Kries chromatic adaptation algorithm. According to the modified von Kries chromatic-adaptation transform, a description of which can be found in Gunter Wyszecki and W. S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae, section 5.12, John Wiley & Sons, Inc., 1982, the L*a*b* color spaces make use of white reference tristimulus data. The modified von Kries chromatic-adaptation transform involves dividing the tristimulus values XYZ obtained for a color produced by a particular color imaging system by white reference tristimulus values for the system. For example, the X, Y, and Z tristimulus values of the color under study can be divided, respectively, by the X, Y, and Z tristimulus values for a perfectly diffuse white reflector. Thus the von Kries approach defines both neutral and chromatic colors relative to the “white reference” representing the XYZ tristimulus values of the perfectly diffuse white reflector. The equations for the CIE 1976 CIELAB color space are as follows:L*=116×f(Y/Yn)−16  [1]a*=500×[f(X/Xn)−f(Y/Yn)]  [2]b*=200×[f(Y/Yn)−f(Z/Zn)]  [3]f(ω)=(ω)1/3ω>0.008856  [4]f(ω)=7.787(ω)+16/116ω≦0.008856  [5]where Xn, Yn, and Zn are the tristimulus values of a perfectly diffuse white reflector under specified viewing conditions. The viewing conditions are determined by (1) the illuminant, e.g., D50, and (2) the Standard Observer (2° or 10°).